Method and system for base station change of packet switched communications in a mobile communications system

ABSTRACT

A mobile station (MS) and method for base station handover from a source cell of a source base station to a target cell of a target base station in a packet-switched cellular radio network. While the MS is still in the source cell, the MS receives from the network, a handover command message and information regarding a next expected uplink packet for a packet flow subject to lossless packet-switched handover. Upon arrival of the MS in the target cell, the MS transmits an uplink sequence number status message to the target base station providing a downlink sequence number status for the packet flow, and starts uplink data transmission to the target base station beginning with the next expected uplink packet.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to communications. More especially itrelates to packet data communications over radio links and change ofbase station. Particularly it relates to base station handover of packetswitched communications in GPRS (General Packet Radio System) and UMTS(Universal Mobile Telecommunications System) communications.

BACKGROUND AND DESCRIPTION OF RELATED ART

Packet Radio Services offers packet switched communications over radiolinks in e.g. GPRS and UMTS. Data is disassembled and transmitted inpackets or Protocol Data Units (PDUs). Upon reception, the PDUs arereassembled.

FIG. 1 illustrates protocol layers for GERAN (GSM-EDGE Radio AccessNetwork) A/Gb mode and will be explained in some detail below. Allfunctions related to transfer of Network layer Protocol Data Units,N-PDUs, shall be carried out in a transparent way by the GPRS networkentities.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup GERAN, Digital cellular telecommunications system (Phase 2+);General Packet Radio Service (GPRS); Overall description of the GPRSradio interface; Stage 2 (Release 4), 3GPP TS 43.064 V4.3.0, France,February 2002, provides the overall description for lower-layerfunctions of GPRS and EGPRS (Enhanced GPRS) radio interface, Um. In thesequel GPRS refers to both GPRS and EGPRS in not explicitly statedotherwise. An EGPRS mobile/base station is a GPRS compliant mobile/basestation with additional capabilities for enhanced radio access protocolfeatures and enhanced modulation and coding schemes. The support ofEGPRS is optional for mobile station and network.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup GSM/EDGE Radio Access Network; General Packet Radio Service(GPRS); Mobile Station (MS)—Base Station System (BSS) interface;RadioLink Control/Medium Access Control (RLC/MAC) protocol (Release 4), 3GPPTS 44.060 V4.8.0, France, September 2002, specifies the procedures usedat the radio interface for the General Packet Radio Service, GPRS,Medium Access Control/Radio Link Control, MAC/RLC, layer. The RLC/MACfunction supports two modes of operation:

-   -   unacknowledged operation; and    -   acknowledged operation.

Section 9.3 describes operation during RLC data block transfer. RLCacknowledged mode, RLC-AM, operation uses retransmission of RLC datablocks to achieve high reliability. RLC unacknowledged mode, RLC-UM,operation does not utilize retransmission of RLC data blocks.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Radio Access Network, Physical Layer Procedures, 3G TS 25.322v3.5.0, France, December 2000, specifies three data transfer services ofradio link control, RLC:

-   -   transparent data transfer service,    -   unacknowledged data transfer service, and    -   acknowledged data transfer Service

Subsections 4.2.1.1 and 4.2.1.2 describe transparent mode entities andunacknowledged mode entities. One difference of the two modes resides inmanagement of packet overhead.

In transparent mode no overhead is added or removed by RLC. Insubsection 4.2.1.3 an acknowledged mode entity, AM-entity, is described(see FIG. 4.4 of the 3GPP Technical Specification). In acknowledged modeautomatic repeat request, ARQ, is used. The RLC sub-layer provides ARQfunctionality closely coupled with the radio transmission techniqueused.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Core Network; Digital cellular telecommunications system (Phase2+); Mobile Station (MS)—Serving GPRS Support Node (SGSN); SubnetworkDependent Convergence Protocol (SNDCP) (Release 5), 3G TS 44.065 v5.1.0,France, September 2003, provides a description of the SubnetworkDependent Convergence Protocol, SNDCP, for GPRS. SNDCP entity performsmultiplexing of data coming from different sources to be sent usingservice provided by the LLC (Logical Link Control) layer,

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Core Network; General Packet Radio Service (GPRS); GPRS TunnellingProtocol (GTP) across the Gn and Gp interface (Release 5), 3GPP TS29.060 V5.8.0, France, December 2003, defines the second version of GTPused on:

-   -   the Gn and Gp interfaces of the GPRS;    -   the Iu, Gn and Gp interfaces of the UMTS.

Within GPRS (and UMTS) Gn interface is an interface between GPRS SupportNodes (GSNs) within a PLMN and Gp interface is an interface between GPRSSupport Nodes (GSNs) of different PLMNs. In UMTS Iu interface is aninterface between RNC and Core Network.

A Gb interface is an interface between an SGSN (Serving GPRS SupportNode) and a BSC (Base Station Controller). An A interface is aninterface between BSC and MSC (Mobile Services Switching Center).

GPRS Tunneling Protocol, GTP, is the protocol between GPRS SupportNodes, GSNs, in the UMTS/GPRS backbone network. GTP allowsmulti-protocol packets to be tunneled through the UMTS/GPRS Backbonebetween GSNs and between SGSN (Serving GSN) and UTRAN (UniversalTerrestrial Radio Access Network).

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Core Network; Mobile Station—Serving GPRS Support Node (MS-SGSN);Logical Link Control (LLC) layer specification; (Release 4), 3GPP TS44.064 V4.3.0, France, March 2002, defines the Logical Link Control,LLC, layer protocol to be used for packet data transfer between theMobile Station, MS, and Serving GPRS Support Node, SGSN. LLC spans fromthe MS to the SGSN. LLC is intended for use with both acknowledged andunacknowledged data transfer.

LLC supports two modes of operation:

-   -   Unacknowledged peer-to-peer operation, LLC-UM, and    -   Acknowledged peer-to-peer operation, LLC-AM.

In unacknowledged operation logical link entity may initiatetransmissions to a peer entity without prior establishment of a logicalconnection with the peer entity. LLC does not guarantee in-orderdelivery. LLC can detect errors in a received frame, and, depending onwhether the frame is sent in protected mode or not, either discard ordeliver the erroneous frame. No error recovery procedures are defined atthe LLC layer. Higher-layer protocols can be used to providereliability, if needed. This mode of operation is known as AsynchronousDisconnected Mode, ADM.

With acknowledged operation a balanced data link involves twoparticipating entities, and each entity assumes responsibility for theorganization of its data flow and for error recovery proceduresassociated with the transmissions that it originates. Each entityoperates as both a data source and data sink in a balanced link,allowing information to flow in both directions. This mode of operationis known as Asynchronous Balanced Mode, ABM, and provides a reliableservice with in-order delivery.

European Patent Application EP1318691 describes a method for informingthe SGSN about a mobile station cell-change operation in the GPRS.

International Patent Application WO03032672 discloses a method ofoptimization of handover procedures in GPRS comprising the old SGSNsending identification response directly to the new SGSN.

International Patent Application WO0079808 claims a method of reducingdelay time for a mobile station being handed over from an old SGSN to anew SGSN during a call handling a real-time payload in a GPRS packetswitched radio telecommunications network comprising shortening theinter-SGSN Routing Area Update interruption interval and implementinglow latency requirements and shaping of packet traffic.

International Patent Application WO02085048 describes a handoverprocedure for use in a GPRS network, reducing the need for re-sequencingin SGSN. Old SGSN sends a message to GGSN(Gateway GSN) requesting datatransmission to stop. Data at old SGSN, for transmission to MS, istransferred to new SGSN and transmission from GGSN is resumed whenhandover is complete. GGSN then transmits data to new SGSN.

In U.S. Patent Application US20010019544 the GGSN and SGSN are allowedto finish up on-going transactions before moving the context to the newSGSN. The first (old) SGSN is operating as a temporary anchor inresponse to an inter-SGSN routing area update.

European Patent Application EP1345463 reveals buffering of TCP packetsin a mobile node during handover.

FIG. 2 illustrates schematically some network elements involved inpacket switched handover. A source SGSN

source SGSN

connected to a gateway GSN

GGSN

supports data traffic to a mobile station

MS

via a source base station subsystem

source BSS

. A base station change may be initiated, e.g. as the mobile stationmoves, towards a base station of a target base station subsystem

target BSS

supported by a target SGSN

target SGSN

.

In prior art lossy type of packet switched handover is used for servicesrequiring short delay but allowing some data loss at cell change, e.g.speech services. For lossy handover downlink data is typicallyduplicated by the source SGSN and sent both to the source BSS forfurther transmission to the mobile station in the current cell, and tothe target SGSN

target SGSN

.

The target side (BSS/SGSN) can either discard the forwarded data untilthe MS has indicated its presence in the target cell or, blindly, sendthe data without information available on whether or not the MS ispresent in the target cell. In case of blindly sending the data, themobile station has been ordered to perform the handover and hassynchronized towards the target cell, the downlink data flow is alreadyongoing and the mobile station can immediately start the uplink dataflow. No acknowledgement of received data is required, neither in uplinknor in downlink.

According to prior art solutions, data losses will occur, e.g., whendata packets sent to source BSS from source SGSN are discarded in sourceBSS when a mobile station is handed over from source BSS to target BSS.Losses will also occur if e.g. packet data forwarded to MS via thetarget SGSN and target BSS experiences a delay that is less than thedelay associated with the MS processing the handover command andacquiring synchronization.

Lossless type of packet switched handover, PS handover, is used forservices that are sensitive to data losses but can accept a certaindelay. The typical characteristics of a lossless handover are currentlybased on acknowledged RLC and LLC protocols and the SNDCP protocoloperating in acknowledged mode. During the PS handover the downlink dataflow is forwarded from the source SGSN to the target SGSN. The targetSGSN buffers the downlink data until the mobile station has indicatedits presence in the target cell. The SNDCP layer at both the MS and SGSNassigns N-PDU Send number to each N-PDU sent and maintains N-PDU Receivenumber for each received N-PDU for any given bi-directional packetservice. When a handover is performed of such a service the number ofthe next expected uplink and downlink N-PDU is exchanged between the MSand the SGSN in handover signaling messages allowing precise knowledgeof where packet data transmission should resume after handover.

None of the cited documents above discloses lossless packet switchedbase station handover or radio cell change in. LLC unacknowledged mode,LLC-UM.

SUMMARY OF THE INVENTION

Packet switched base station handover according to the invention isassociated with cell change both within GERAN and between GERAN andUTRAN. According to the invention the cell reselection time can bereduced. Also, due to the invention allowing operating, particularlyLLC, in unacknowledged mode, link delay can be reduced during an entiredata transfer session. This is particularly the case for base stationhandover in RLC-UM and LLC-UM.

Handover for which data losses may occur is known as lossy handover. Forpacket data communications with strict delay requirements losslesshandover according to prior art is not always feasible due to itsimposed additional delay caused by acknowledgements and retransmissions.The delay introduced by particularly LLC protocol layer in acknowledgedmode affects higher layer operations, e.g. TCP based services with aresulting throughput deterioration due to TCP congestion controlerroneously interpreting the additional delay as channel congestion.Presently the only packet switched handover solution offered for delaysensitive applications, when LLC-AM is excluded, is lossy handover.Lossless packet switched handover can, according to prior art, only beachieved by operating LLC/SNDCP in acknowledged mode, which willincrease overhead, add delay and reduce overall throughput.

Consequently, there is a need of reducing data transfer delay andcontrol signaling, without risking packet losses due to packet switchedhandover.

It is consequently an object of the present invention to reduce datatransfer delay without packet losses due to handover.

A further object is to reduce packet switched data transfer overheadwithout packet losses due to handover.

It is also an object not to require LLC-AM as a means for reducing therisk of data losses at packet switched handover.

Another object is to circumvent throughput reduction due to handover.

Finally, it is an object of the present invention to enhance LLC/SNDCPprotocols to provide lossless handover with LLC/SNDCP operating inunacknowledged mode.

These objects are met by a method and system of lossless base stationhandover for packet switched communications not requiring LLC/SNDCP tooperate in acknowledged mode during the complete data transfer session.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates protocol layers for GERAN (GSM-EDGE Radio AccessNetwork) A/Gb mode, according to prior art.

FIG. 2 illustrates schematically some network elements involved inpacket switched handover.

FIG. 3 illustrates an outline of a signaling diagram associated with anexample PS handover according to a mode of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

To handle handover of packet services that require a minimum of packetloss it is currently possible to operate both the RLC and the LLC/SNDCPprotocols in acknowledged mode. However this is not desirable as withthese two protocols operating in acknowledged mode a certain amount ofdelay will be introduced whenever retransmission is determined to benecessary at any of these layers. The delay introduced especially whenretransmission occurs at the LLC layer may impact higher layeroperation, e.g. for TCP based services where a protocol stack consistingof TCP/IP/SNDCP/LLC/RLC is used, with the net result being a majorreduction in throughput temporarily being experienced by the affectedpacket service. This extra delay will impose a lowered quality ofservice as perceived by the user. In addition, operating these twoprotocols in acknowledged mode will result in increased overhead usedfor control plane functions, and therefore renders less bandwidthavailable to user plane payload.

Today's streaming services are normally implemented operating RLC inacknowledged mode and LLC/SNDCP in unacknowledged mode, which allows foreliminating the potential for serious delay problems as described above.However, this approach has the problem of not being able to supportlossless packet service for the case where MS mobility involves a changeof radio cell/base station and SGSN.

Packet switched base station handover according to the invention isassociated with radio cell change both within GERAN and between GERANand UTRAN.

To minimize potential delay and extra overhead as would result ifoperating the LLC/SNDCP protocols in acknowledged mode, for all packetflows subject to lossless packet-switched handover, PS handover, and tobe compliant with the principles used for lossless data transfer inUTRAN, the following embodiments are identified:

-   -   new mode of operation for SNDCP    -   management of downlink Status with/without Source BSS        assistance, and    -   management of uplink status with/without source BSS assistance.

They are preferably combined. The packet loss during handover isminimized without requiring LLC/SNDCP protocols to operate inacknowledged mode during an entire data transfer session. Thereby higheruser data rates are achieved during the entire data transfer session.

New Mode of Operation for SNDCP

The SNDCP protocol is modified to support a new mode of operation whereit operates with both N-PDU Send and Receive Sequence numbers combinedwith the LLC protocol operating in unacknowledged mode. This means thatthe SNDCP protocol entities in the mobile station and in the networkshall each maintain a Send and a Receive N-PDU Sequence number and alsoGTP T-PDU uplink and downlink sequence numbers for each packet flowsubject to lossless PS handover. This sequence number information isforwarded from the source SGSN to the target SGSN so that a SNDCP enginestarted in the target SGSN can maintain sequence number continuity withthe SNDCP engine used in the source SGSN. The downlink N-PDU sequencenumber and the downlink GTP T-PDU numbers are provided along with eachN-PDU forwarded from the source SGSN to the target SGSN.

Management of Downlink Status With Source BSS Assistance

The downlink LLC data buffered in the source BSS that has not yet beensent to or acknowledged by the mobile station (at the RLC layer) at thepoint of time when the source BSS sends the PS handover command messageto the MS can be deleted and a status message sent back to the sourceSGSN telling it how many LLC PDUs were deleted for each packet flowsubject to lossless PS handover.

Alternatively, the status message sent from the source BSS to the sourceSGSN could provide parts of the deleted LLC PDUs, e.g. the LLC header,or even the complete LLC PDUs. This means that SN-UNITDATA PDUspreviously sent down to LLC at the source SGSN and relayed to the sourceBSS as segmented LLC PDUs will either be explicitly returned to thesource SGSN or referenced in such a way to allow the source SGSN todetermine which N-PDUs have not been sent to the MS, i.e. which wholeN-PDUs have been acknowledged by the MS on RLC layer.

The Send N-PDU sequence numbers determined by the source SGSN are thenforwarded in a message to the target SGSN. Upon MS arrival in the targetcell the target SGSN can start transmitting the next downlink N-PDUexpected by the MS for each packet flow subject to lossless PS handover.The MS can detect duplications of downlink N-PDUs since the sequencenumber used at the target SGSN is based on the sequence number used bysource SGSN and it is included in the header of each N-PDU sent from thetarget SGSN.

For this embodiment it is required that the source SGSN supports N-PDUbuffering if downlink N-PDUs are to be forwarded to the target SGSN intheir correct order or the source BSS sends a status message that doesnot contain complete LLC PDUs. If the target SGSN can accept N-PDUs thatare out of order and the status message sent by the source BSS containscomplete LLC PDUs then the source SGSN need not support N-PDU buffering.

Management of Downlink Status Without Source BSS Assistance

In this embodiment the source SGSN can only estimate the Send N-PDUsequence numbers based on the delay attribute (with sufficient marginadded) associated with each packet flow subject to lossless PS handoverand forward these estimates to the target SGSN.

Source BSS buffers has downlink LLC data not yet sent to or acknowledgedby the mobile station (at the RLC layer) at the point of time when thesource BSS sends the PS handover command message to the MS. The sourceBSS sends the PS handover command message to the MS. Thereafter it sendsa status message back to the source SGSN only indicating that the MS hasbeen sent a PS handover command message (i.e. no downlink status isincluded in the message).

If upon arrival in the target cell, the MS does not send the network amessage that provides downlink sequence number status for all packetflows subject to lossless PS handover, the target SGSN has no choice butto use the estimated Send N-PDU sequence numbers provided by the sourceSGSN. This may result in the target SGSN sending multiple downlinkN-PDUs already received by the MS. However, the MS can once again detectduplications of downlink N-PDUs since the sequence number is included inthe header of each SN-UNITDATA PDU used to transit each N-PDU.

If upon arrival in the target cell, the MS sends the network a messagethat provides downlink sequence number status for all packet flowssubject to lossless PS handover, the target SGSN will have exactknowledge as to which downlink N-PDU to begin sending for each packetflow. The target SGSN deletes all downlink N-PDUs forwarded from thesource SGSN that are implicitly acknowledged by the downlink sequencenumber status provided by the MS upon arrival in the target cell.

The embodiment requires that the source SGSN supports N-PDU bufferingsince a minimum set of downlink N-PDUs sent down to the source BSS mustbe buffered for each packet flow subject to lossless PS handover. Thequantity of N-PDUs buffered for a given packet flow can be determinedby, e.g., the delay attribute associated with that packet flow.

Management of Uplink Status With Source BSS Assistance

When the source BSS receives the PS handover command message from thesource SGSN it will at a point of time stop acknowledging RLC packets inthe uplink. When this occurs it will send a status message to the sourceSGSN indicating that no more uplink LLC PDUs will be forwarded to it.The source SGSN can then determine the Receive N-PDU sequence numbersfor all uplink packet flows subject to lossless PS handover and includethem in a message to the target SGSN. After notifying the source SGSN,the source BSS will send the PS handover command message to the MS.

The PS handover command message may contain an up to date RLC ACK/NACKreport allowing the MS to determine which N-PDUs have been completelyreceived by the network. The MS will start uplink transmission uponarrival in the target cell from the next uplink N-PDU that was notacknowledged by lower layers in the old cell. This N-PDU should alwayscorrespond to the next uplink N-PDU expected by the target SGSN for eachpacket flow subject to lossless PS handover.

Alternatively, the PS handover command message may not include an RLCACK/NACK report or any other indication of uplink status that the MScould use to determine the Send N-PDU sequence number for the packetflows subject to lossless PS handover. The MS will therefore startuplink transmission upon arrival in the target cell from what isestimated next uplink N-PDU that was not acknowledged by lower layers inthe old (source) cell. In this case the first N-PDU sent by the MS inthe new cell may not correspond to the next uplink N-PDU expected by thetarget SGSN. However, since the N-PDU sequence number is included in theheader of each SN-UNITDATA PDU used to transmit each N-PDU the targetSGSN will be able to remove any duplication.

For both alternatives the source BSS is considered to have providedassistance to the source SGSN in that a status message is sent. Themessage indicates that the source BSS has stopped acknowledging RLCpackets in the uplink and that no more uplink LLC PDUs will be forwardedto it.

Management of Uplink Status Without Source BSS Assistance

The PS handover command message may be sent from the source SGSN to thesource BSS and include the expected Receive NPDU sequence Number thatthe MS should start transmission with in the target cell for each uplinkpacket flow subject to lossless handover. This sequence numberinformation is provided by the source SGSN without conferring with thesource BSS as to whether or not it has stopped acknowledging RLC data inthe uplink. As such, additional uplink LLC PDUs may be acknowledged bythe source BSS prior to the PS handover command message being sent tothe MS and may therefore result in a conflict with the Send N-PDUsequence number as viewed by the MS. In this case the MS must alwaysaccept the uplink sequence number information provided in the PShandover command message over the uplink sequence number informationderived from local RLC operation. This means that the MS must alwaysbuffer some uplink N-PDUs that have already been confirmed according toRLC. The quantity of these N-PDUs that need to be buffered is expectedto be small.

According to the invention a sequence tracking mode, STM, is defined forSNDCP. In STM, the SNDCP entities in the SGSN and in the MS shall alwayskeep track of the uplink and downlink N-PDU sequence numberscorresponding to SNDCP in LLC-AM. Further, in STM uplink and downlinkG-PDU sequence numbers associated with uplink and downlink N-PDUs arerecorded corresponding to SNDCP in LLC-AM. Also, in STM SNDCP entitiesin SGSN and in MS shall make use of SN-UNITDATA PDUs corresponding toSNDCP in LLC-UM and SNDCP shall maintain sequence number continuity whenPS handover occurs across SGSN.

An additional case is where the PS handover command sent to the MS doesnot include any Receive N-PDU numbers for any of the uplink packet flowssubject to lossless PS handover. In this case the MS will use its localknowledge of uplink status which may lead to a duplicated uplink N-PDUbeing sent by the MS in new (target) cell. Since this duplication willbe deleted by the target SGSN, since N-PDU sequence number continuity issupported across SGSNs, it will not be a problem.

FIG. 3 illustrates an outline of a signaling diagram associated with anexample PS handover according to a mode of the invention. In the exampleLLC operates in unacknowledged mode LLC-UM, even if the invention isapplicable also in acknowledged mode, LLC-AM.

The signaling of FIG. 3 is initiated by an MS having one or more ongoingpacket flows subject to lossless PS handover when the source BSSdetermines that a PS handover is required

1

. RLC is operating in acknowledged mode, LLC is operating inunacknowledged mode and SNDCP is operating in sequence tracking mode,STM. Thereby N-PDU sequence numbering is managed as if LLC wereoperating in acknowledged mode. Therefore SNDCP entities in MS andnetwork has to manage two sequence parameters for each packet flowsubject to lossless PS handover, the Send N-PDU and the Receive N-PDUsequence numbers.

In STM SNDCP shall use SN-UNITDATA PDUs as with LLC-UM. For each packetflow subject to lossless PS handover the source SGSN buffers a set ofdownlink N-PDUs that reflects the delay attribute associated with thatpacket flow. As a non-exclusive example the N-PDUs received from theGGSN during the running latest 500 ms are buffered.

The source BSS sends a PS Handover Required message

2

to the source SGSN.

The source SGSN sends a Prepare PS Handover Request message to thetarget SGSN

3

.

The target SGSN sends a PS Handover Request message

4

to the target BSS. The target BSS pre-allocates radio resources, ifavailable, to the requested flows and sends a PS Handover RequestAcknowledge message

4′

back to the target SGSN.

The target SGSN sends a Prepare PS Handover Response message to thesource SGSN

5

. This message indicates that the SGSN is now ready to receive downlinkdata forwarded from the source SGSN. When source SGSN receives thePrepare PS Handover Response message

5

it

-   -   stops sending downlink data to the source BSS,    -   sends the PS Handover Command message        6        to the source BSS containing among other things the N-PDU        Receive Sequence number of the next expected uplink N-PDU to be        received for each packet flow subject to lossless PS handover,    -   starts forwarding to the target SGSN all buffered downlink        N-PDUs received from the GGSN prior to the arrival of the        Prepare PS Handover Response message from the target SGSN, and    -   starts forwarding to the target SGSN downlink N-PDUs received        from the GGSN after the arrival of the Prepare PS Handover        Response message from the target SGSN        9        .

Each downlink N-PDU forwarded to the target SGSN

9

contains an associated Send N-PDU sequence number and a GTP sequencenumber. The target SGSN starts buffering of the forwarded downlinkN-PDUs until the MS indicates its presence

7

in the target cell by sending a PS Handover Complete message to thetarget SGSN

7

,

10

.

When the source BSS receives the PS Handover Command

6

it

-   -   immediately stops reception and acknowledgement of data in the        uplink;    -   stops transmission of downlink data towards the MS but may        terminate transmission at an LLC PDU boundary without waiting        for an acknowledgement;    -   sends a Forward BSS Context message to the source SGSN, the        message not including information on which buffered downlink        LLC-PDUs has been discarded;    -   sends a PS Handover Command        7        to the MS ordering the MS to a new target cell; the message        including Receive N-PDU (uplink) sequence number of the next        expected N-PDU to be received as viewed by the source SGSN for        each packet flow subject to lossless PS handover;

When the MS has reconfigured itself and acquired synchronization in thenew cell, it sends a PS Handover Complete message to the target BSS

7′

. This message includes the sequence number of the next expecteddownlink N-PDU to be received (as viewed by MS) for each downlink packetflow subject to lossless PS handover.

The source BSS then sends a Forward BSS Context message

8

to the source SGSN indicating that the BSS has ordered the MS to the newcell. The message comprises no send buffer status information that thesource SGSN could make use of for determining the precise status oftransmitted downlink N-PDUs before starting forwarding data to thetarget SGSN.

Upon receiving the Forward BSS Context message

8

, the source SGSN determines the following values for each packet flowsubject to lossless PS handover and forwards this information to thetarget SGSN in the Forward SRNS Context message

9

:

-   -   Downlink N-PDU Send sequence number for the next downlink N-PDU        to be sent to the MS,    -   Downlink GTP-U sequence number for the next downlink GTP-U T-PDU        to be relayed to the target SGSN,    -   Uplink N-PDU Receive sequence number for the next uplink N-PDU        to be received from the MS, and    -   Uplink GTP-U sequence number for the next uplink GTP-U T-PDU to        be sent from the target SGSN to the GGSN.

Prior to receiving the Prepare PS Handover Response message

5

the source SGSN was buffering downlink N-PDUs according to the delayattributes of the packet flows subject to lossless PS handover. Sincethe Forward BSS Context message

8

received from the source BSS does not include downlink statusinformation, the source SGSN selects values for the two downlinksequence numbers listed above that reflect the oldest buffered downlinkN-PDU for each of the packet flows subject to lossless PS handover. I.e.a worst case scenario is anticipated and corresponding values selected.

Upon receiving the Forward SRNS Context message, the target SGSN sends aForward SRNS Context Acknowledge message

9′

back to the source SGSN.

Thereafter the target BSS sends a PS Handover Complete message

10

to target SGSN. The PS Handover Complete message

10

includes the sequence number of the next expected downlink N-PDU to bereceived (as viewed by MS) for each packet flow subject to lossless PShandover.

The target SGSN can now start sending the buffered downlink datastarting with the next downlink N-PDU expected by the MS for each packetflow subject to lossless PS handover. The downlink sequence numberstatus information provided in the PS Handover Complete message

12

allows the target SGSN to:

-   -   delete all downlink N-PDUs forwarded from the source SGSN that        are implicitly acknowledged by the downlink sequence number        status information, and    -   ignore downlink sequence number status information provided by        the source SGSN in the Forward SRNS Context message        9        .

The target SGSN sends a PS Handover Complete message

12

to the source SGSN, which acknowledges the completion of the handoverprocedure by responding with a PS Handover Complete Acknowledge message

12′

back to target SGSN. Target SGSN sends an Update PDP Context Request tothe GGSN

13

. The GGSN updates its PDP context fields and return an Update PDPContext Response message

13′

. SGSN initiates Packet Flow Procedures to release resources in thesource BSS

14

. Finally, MS and target SGSN perform routing area update procedure

15

.

The example signaling described above in relation to FIG. 3 illustratesa method and system where source SGSN provides the MS with the sequencenumber of the next expected uplink N-PDU to be received in the PSHandover Command

6

,

7

. (Management of uplink status is provided without informationprocessing of source BSS.)

MS preferably provides the network with the sequence number of the nextexpected downlink N-PDU to be received in the PS Handover Completemessage

7′

,

10

. (Management of downlink status is provided without informationprocessing of source BSS).

SNDCP entities in the source SGSN preferably support some buffering ofdownlink N-PDUs. The source SGSN then buffers an amount of N-PDUscorresponding to the delay attribute of the associated packet flow. Uponcompletion of the PS handover preparation phase all such buffered N-PDUscan be forwarded to the target SGSN to ensure all forwarded N-PDUsarrive in correct order. Downlink N-PDUs received from the GGSN after PShandover preparation is completed will be sent to the target SGSN afterthe buffered downlink N-PDUs are forwarded. Upon MS arrival to the newcell, the target SGSN discovers the downlink status of the packet flows,e.g. via information provided in the PS Handover Complete message

10

, begins transmitting the appropriate downlink N-PDU for each packetflow subject to lossless PS handover and deletes all implicitlyconfirmed downlink N-PDUs received from the source SGSN.

In SNDCP STM, SNDCP entities in the target SGSN will be required tosupport some buffering of downlink N-PDUs. This is necessary as losslessoperation requires that the target SGSN be informed of the presence ofthe MS in the new cell before it can begin downlink transmission ofpacket data.

The source BSS also supports buffering which allows it the option ofattempting to empty downlink buffers upon receiving a PS HandoverCommand

6

from the source SGSN.

In SNDCP STM, SNDCP entities in MS will be required to buffer the uplinkN-PDUs beyond the point where their associated RLC/MAC entitiesacknowledges the complete transmission of any given LLC PDU. This isnecessary for the example signaling where management of uplink status isprovided without information processing of source BSS. This bufferingallows source BSS the option of continued reception of uplink data whenattempting to empty downlink buffers upon reception of a PS HandoverCommand

6

from the source SGSN.

The example described in relation to FIG. 3 is just an example. Itillustrates, e.g., source SGSN and target SGSN as separate entities(inter SGSN PS handover). However, the invention also covers intra SGSNPS handover between base stations. Also, in FIG. 3 signaling for onesingle radio access technology, RAT, is illustrated. Though, the sameprinciples are valid also for inter RAT PS handover, such as for PShandover between base stations of GERAN and UTRAN respectively.

FIG. 4 illustrates a simplified block diagram of a mobile stationaccording to the invention. The mobile station comprises processingmeans

μ_(MS)

operating according to one or more protocols for communicating protocoldata units as described above. Receive means

R_(MS)

receives information from the communications network to which the mobilestation is connected. The receive means are connected to the processingmeans

μ_(MS)

and receives, e.g., information from the communications network on nextexpected uplink protocol data unit at handover. Buffer means

B_(MS)

buffers uplink protocol data units, N-PDUs, as described above.

FIG. 5 displays schematically a block diagram of a support node, such asa Serving GPRS Support Node, SGSN, according to the invention. Thesupport node comprises processing means

μ_(SN)

operating according to one or more protocols for communicating packetswitched data as described above. Receive means

R_(SN)

receives protocol data units from one or more respective mobile stationson next expected downlink protocol data unit, N-PDU. Transmit means

T_(SN)

transmits protocol data units to the one or more mobile stations whichare communicating packet switched data over the SGSN. Buffer means

B_(SN)

buffers downlink N-PDUs.

FIG. 6 depicts a schematic block diagram of a base station entityaccording to the invention. The base station entity comprises receivemeans

R_(BS)

, transmit means

T_(BS)

and buffer means

B_(BS)

. The receive means

R_(BS)

receiving one or more commands of base station change as decided bynetwork to which the base station entity is connected. Also, receivemeans receives

R_(d,BS)

that is not necessarily identical to receive means

R_(BS)

, receives uplink data from one or more mobile stations communicatingpacket switched data via the base station entity. The transmit means

T_(BS)

transmits protocol data units to one or more mobile stationscommunicating packet switched data via the base station entity.

A person skilled in the art readily understands that the properties ofan SGSN, a GGSN, a BSS, a base station or an MS are general in nature.The use of concepts such as SGSN or MS within this patent application isnot intended to limit the invention only to devices associated withthese acronyms. It concerns all devices operating correspondingly, orbeing obvious to adapt thereto by a person skilled in the art, inrelation to the invention. As an explicit non-exclusive example theinvention relates to mobile equipment without a subscriber identitymodule, SIM, as well as mobile stations including one or more SIMs.Further, protocols and layers are referred to in close relation withGPRS, UMTS and Internet terminology. However, this does not excludeapplicability of the invention in other systems with other protocols andlayers of similar functionality.

The invention is not intended to be limited only to the embodimentsdescribed in detail above. Changes and modifications may be made withoutdeparting from the invention. It covers all modifications within thescope of the following claims.

1-177. (canceled)
 178. A method in a mobile station (MS) for basestation handover from a source cell of a source base station to a targetcell of a target base station in a cellular radio network, wherein thebase stations transfer packet switched communications between the mobilestation and the network, the method comprising the steps of: while theMS is still in the source cell: receiving from the network, a handovercommand message ordering the MS to connect to the target cell; andreceiving from the network, information regarding a next expected uplinkpacket for a packet flow subject to lossless packet switched handover;and upon arrival of the MS in the target cell, transmitting an uplinksequence number status message to the target base station providing adownlink sequence number status for the packet flow subject to losslesspacket switched handover, and starting uplink data transmission to thetarget base station beginning with the next expected uplink packet. 179.The method according to claim 178, wherein the mobile station alsoreceives a transfer of radio link control data blocks in an acknowledgedmode.
 180. The method according to claim 178, wherein the sequencenumber status message transmitted to the target base station includes asequence number of the next expected Protocol Data Unit (PDU) to bereceived by the mobile station.
 181. The method according to claim 178,wherein the mobile station receives lower layer packet acknowledgements,and the step of starting uplink data transmission includes startinguplink data transmission in the target cell beginning with the nextuplink packet that was not acknowledged by lower layers in the sourcecell.
 182. A mobile station (MS) configured to be handed over from asource cell of a source base station to a target cell of a target basestation in a cellular radio network, wherein the base stations transferpacket switched communications between the mobile station and thenetwork, the mobile station comprising: a receiver configured to:receive, while the MS is still in the source cell, a handover commandmessage from the network ordering the MS to connect to the target cell:and receive, while the MS is still in the source cell, informationregarding a next expected uplink packet for a packet flow subject tolossless packet switched handover; and a transmitter configured to:transmit an uplink message to the target base station when the mobilestation arrives in the target cell, the uplink message providing adownlink sequence number status for the packet flow subject to losslesspacket switched handover; and start uplink data transmission to thetarget base station beginning with the next expected uplink packet. 183.The mobile station according to claim 182, wherein the mobile station isconfigured to receive a transfer of radio link control data blocks in anacknowledged mode.
 184. The mobile station according to claim 182,wherein the mobile station is configured to include in the uplinkmessage, the next expected Protocol Data Unit (PDU) to be received. 185.A method in a mobile station (MS) for base station handover from asource cell of a source base station to a target cell of a target basestation in a cellular radio network, wherein the base stations transferpacket switched communications between the mobile station and thenetwork, the method comprising the steps of: while the MS is still inthe source cell, receiving from the network, a handover command messageordering the MS to connect to the target cell; and upon arrival in thetarget cell, starting uplink data transmission to the target basestation beginning with a next expected uplink packet.
 186. The methodaccording to claim 185, wherein the mobile station receives lower layerpacket acknowledgements, and the step of starting uplink datatransmission includes starting uplink data transmission in the targetcell beginning with the next up-link packet that was not acknowledged bylower layers in the source cell.
 187. The method according to claim 185,further comprising, after receiving the handover command message, thestep of estimating by the MS, the next expected uplink packet for apacket flow subject to lossless packet switched handover, wherein theestimated next expected uplink packet is the first uplink packet sent bythe MS in the source cell that was not acknowledged in the source cell.188. The method according to claim 185, wherein the step of starlinguplink data transmission includes starting the uplink data transmissionbefore receiving any sequence number status message from the target basestation.
 189. A mobile station (MS) configured to be handed over from asource cell of a source base station to a target cell of a target basestation in a cellular radio network, wherein the base stations transferpacket switched communications between the mobile station and thenetwork, the mobile station comprising: a receiver configured to receivea handover command message from the network ordering the MS to connectto the target cell; and a transmitter configured to transmit uplinkpackets to the target base station beginning with a next expected uplinkpacket.
 190. The mobile station according to claim 189, wherein themobile station receives lower layer packet acknowledgements, and thetransmitter is configured to start uplink data transmission in thetarget cell beginning with the next up-link packet that was notacknowledged by lower layers in the source cell.
 191. The mobile stationaccording to claim 189, wherein the processor is further configured toestimate the next expected uplink packet for a packet flow subject tolossless packet switched handover, wherein the estimated next expecteduplink packet is the first uplink packet sent by the MS in the sourcecell that was not acknowledged in the source cell.
 192. The mobilestation according to claim 189, wherein the transmitter is configured tostart the uplink data transmission before receiving any sequence numberstatus message from the target base station.